Frequently independent antennas, such as spiral antennas, are well known in the antenna industry. They are often used in applications where it is desirable to have a radiation pattern and impedance which remain fairly constant over a wide range of frequencies. An important characteristic of this planar type of antenna is its complementary structure. Between the strips of metal forming the arms of the spirals are strips of non-metal material. If the metal part of the antenna is the same as the non-metal part except for the angle of rotation, the antenna is said to be self-complementary. Self-complementary antennas of infinite extent have frequency-independent impedances.
Although the impedance, radiation pattern and polarization of spiral antennas generally remain fairly constant over a wide range of frequencies, factors such as the overall radius of the antenna, the flare rate of the spirals, and the feed point of the antenna affect performance. Generally, the low-frequency end of the operating band is imnited by the overall radius of the antenna while the high-frequency end is limited by the feed structure. Therefore, these factors, among others, must be taken into account when designing a spiral antenna.
A properly designed spiral antenna is a member of the class of self-complementary planar antennas, which have an inherently constant impedance that is virtually frequency independent. The spiral-mode microstrip (SMM) antenna, which generally comprises a self-complementary planar antenna having a ground plane added to one side thereof, represents a major improvement that allows the antenna to be compatible, mountable and conformable with any surface on which the antenna is to be placed. The SMM antenna is disclosed in U.S. Pat. No. 5,313,216, issued May 17, 1994 to J. J. H. Wang and V. K. Tripp, which is incorporated herein by reference.
The conformability of an antenna, i.e., the flexibility of an antenna to be molded to a surface having an arbitrary shape without being adversely affected in its performance, is an important factor to be considered when selecting or designing an antenna because antennas are often mounted on the surface of cars, aircraft, human body, etc.
An important class of antennas are phased arrays. Phased arrays have many antenna elements which can be phase shifted to produce beam steering. Phased arrays have two fundamental components, namely, the antenna elements and the phase shifters. Desirable features of phase shifters include fast switching speed, low insertion loss, wide bandwidth, and large power-handling capability. Electronic phase shifters are invariably narrow-band. It has long been recognized that there is a phase pattern associated with the wide-band spiral antenna. If one rotates a spiral antenna mechanically, a frequency-independent phase shift equal to the angle of mechanical rotation can be realized. As a result, a spiral antenna can simultaneously serve as both a radiating element and a phase shifter. Based on this principle, implementing spiral antennas as phased array elements began in the 1950s. These early phased arrays were based on electromechanical rotation to control the phase shift of each antenna element to steer the beam of a phased array comprised of a number of spiral antennas.
During the 1970s, attempts were made to develop electronically-steered "reflectarrays" which were steered by electronically switching the elements of the spiral antenna off and on to electrically rotate the reflected wave from the spiral. This was accomplished through the use of diodes which connected pairs of arms of the spirals in a short-circuit or open-circuit configuration depending on whether the diode is on or off. These arrays were discussed extensively in a four-part series in Microwave Journal (1976-1977) by H. R. Phelan. Although it was proclaimed that these arrays had the potential for low cost, low weight and frequency independence, these antennas did not exhibit broadband characteristics. This puzzling lack of bandwidth contrary to their theory and claim was later explained by Dr. Johnson J. H. Wang in "Characteristics of a New Class of Diode-Switched Integrated Antenna Phase Shifter", IEEE TRANSACTIONS ON ANTENNAS AND PROPOGATION, VOL. AP-31, NO. 1, January 1983. In this article, Dr. Wang demonstrated that a spiraphase antenna, when operating as a reflector in the spiraphase, does not have the broadband characteristics normally associated with the spiral antenna. Dr. Wang pointed out that, for broadband operation, the spiral must operate directly as a transmit and/or receive antenna rather than as a reflector in Phelan's design, but did not indicate how this could be accomplished.
However, Dr. Wang's approach, which is the invention disclosed in the present application, is difficult to implement. A multi-arm spiral must be connected to a feed network that divides a signal into N components (N being the number of spiral arms), with the n.sup.th component feeding the n.sup.th arm (n=1, 2, . . . N) having an equal amplitude but a relative phase of 2.pi.m (n-1)/N radians, or 360 (n-1) m/N degrees, where m is the mode number of the spiral excitation. For example, the feed network of a 4-arm mode-1 spiral must be able to provide inputs of equal amplitude but 0.degree., 90.degree., 180.degree. and 360.degree. in phase to the four spiral arms in a clockwise or counterclockwise manner (for right-hand or left-hand circular polarization). Since the wideband phase shifter is difficult to implement, the practical advantage of the integrated antenna phase shifter would greatly diminish if a wideband phased feed network was required to be employed in the design. Therefore, a technique is needed which allows the basic concept of the integrated antenna phase shifter to be realized and which is practical and efficient.
The integrated antenna phase shifter of the present invention can be used, in addition to those systems using phased arrays, as an antenna and modulator for telecommunications. In particular, the antenna phase shifter of the present invention provides a digital phase switching feature which is directly applicable to digital modulation using phase shift keying (PSK) techniques such as binary PSK, quadrature PSK, etc.